Force transducers are often used as force-to-frequency converters in accelerometers and other instruments. One type of force transducer employs a vibratable assembly which can be used to sense acceleration. In one known arrangement, the transducers are used in push-pull pairs in which a given acceleration results in a compression force on one transducer, and a tension force on the other transducer. This mode of operation provides a high degree of compensation for many so-called common mode errors, i.e. errors that cause the frequencies of the transducer to shift by the same amount in the same direction, because the shifts cancel in the algorithms normally used to process the transducer outputs. Such errors include vibration rectification errors, errors induced by temperature change, aging errors, and measurement errors induced by a drift in the clock frequency.
Such force transducers can also be sensitive to density or pressure variations. The density or pressure sensitivity is primarily due to mass loading effects on the beams. Specifically, gas molecules near the beams tend to oscillate with the beams. Such effectively increases the mass of the beams, thereby affecting the frequencies at which the beam vibrates. When the pressure or density of the surrounding gas increases, the effective mass of the beams also increase which lowers the beams' vibration frequencies. Because the beams' vibration frequencies are employed as a measurement of the applied force, e.g. acceleration, the density-induced or pressure-induced variation can cause an unwanted error in the sensed acceleration output. Accordingly, it would be desirable to reduce, if not eliminate this density-induced or pressure-induced error.
Precision force transducers can be packaged in a vacuum to avoid errors resulting from the density-induced variation. However, the choice of internal materials used in precision sensor designs is severely constrained due to out-gassing concerns. Because gas density within the package directly drives density-induced variations, a heavy burden of hermeticity is place on the packaging. The material constraints in hermeticity requirements, in turn, increase the cost and limit performance. Accordingly, it would be desirable to provide an accelerometer with reduced pressure effects and simpler packaging constraints.
This invention arose out of concerns associated with improving forced-sensing transducer and accelerometer operations. In particular, the invention arose out of concerns associated with providing improved vibrating beam force transducers and methods.